For that matter, what about Key West?
I've snorkled in two locations in Key West.
Once on the patch reefs about 15 miles out. Not much coral to speak of but some really cool fish. Lots of great barracuda (one I swear was 5 ft long!), saw a french angel and my personal fave was a big group of reef squid. They just hovered below us and pulsed in a really cool color pattern. Then they just disappeared and a big barracuda came cruising by. lol
Next we took a boat to Fort Jefferson in the Dry Tortugas National Park. A small chain of tiny unpopulated islands. If you go to Key West you MUST do this. It's a day trip, about 3 hours west into the Gulf of Mexico. The fort was used in the Civil War to house federal prisoners. There is a moat around the fort that you can snorkel on the outside of and it's fantastic. Crystal clear water and tons and tons of life. Huge schools of fish, big queen angels, groupers, lots of coral etc. It was great. Cost about $100 bucks and was well worth it.
Only problem with this time of year, there are too many Man-O-Wars in the water so be careful! I'll be there in May, can't wait!